As suggested by PSP and "The Mystical Seeker of Caves", rearranging your harness to tuck one or two side bottles (as appropriate) will help a lot.
When I changed over to a backplate and harness on my Yellow Box of Debt, I put an SS 45 degree D-ring just below each shoulder, and a 90 degree D-ring just aft of each hip-bone. With an SS dog-leash clip (bolt-snap, insert favorite term, whatever) at the neck ring, and one about a third of the way up from the base of the cylinder, they stay tight to the body as PSP suggested. Use the longer, and thinner tanks for less gas, as opposed to short fat ones. Just clip on what you need and go. If you are not using side-tanks on a particular dive, the D-rings are handy for anything else, such as camera attachment points, etc.
Just a suggestion. BJD :anakinpod